1. Field of the Invention
The present invention relates to a projection lens, and is suitably applied to a projection lens equipped to a projection device such as a projection display device.
2. Description of the Related Art
Recently, a projection display device has been widely used.
A projection display device as described above is known as a so-called back projection type projection display device and performs a display operation by projecting image light to a transmission type screen from the back side thereof.
In such a back projection type projection display device, a beam of light obtained by collimating light of a white light source by a reflector or the like is decomposed into three-color light of red, green and blue components by a color-separating mirror.
The light beams of three colors are each incident on a two-dimensional image display element, for example, a Liquid Crystal Display (LCD), which is formed according to a video electrical signal of red, green, blue (R, G, B). Image light obtained on the respective two-dimensional image display elements corresponding to red, green and blue is color-composed into white color in a color composing optical system, and projected onto a transmission type screen through a projection lens while being enlarged.
A large number of wide-angle type photographic lenses of the kind used for single-lens reflex cameras, each having a long back focus, have been proposed as wide-angle projection lenses for CRT-based projection televisions.
Further, there is a case where the projection display device has a structure that bends the an optical path by 90.degree..
With this structure, the arrangement direction of the housing of a projection display device and the mounting direction of various optical elements, from the color separation to the color composition, can be changed, and further the various optical elements can be miniaturized, whereby miniaturization of the projection display device is made possible.
In the construction of the projection display device as described above, a so-called back focus, which corresponds to the distance from the two-dimensional image display element to the rear end of the projection lens, must nescessarily be a long distance because of the necessity of arranging the optical elements, such as a dichroic prism, a dichroic mirror or the like.
Further, when an enlarged image is formed on the overall transmission type by a projection display device, it is required to shorten the projection distance, that is, the central light beam length extending from the emission end of the projection lens through the mirror to the transmission type screen, in order to miniaturize the projection display device itself. In order to satisfy this requirement, it is necessary that the projection lens have a wider angle and the divergence angle of the emitted light is increased, thereby resulting in a large overall size. Further, in order to transmit light from a light source through the two-dimensional image display element and display an image with high contrast on a screen while the image is enlarged, the beams of light emitted from the two-dimensional image display element must be at an angle that is nearly vertical.
Still further, in order to reduce nonuniformity of color on the screen onto which the image is projected, it is desirable to make constant the angle width of the light beams impinging on the coat face of a dichroic prism or dichroic mirror.
Accordingly, it is necessary that the projection lens has telecentricity so that the principal ray on the axis of the projection lens is vertical relative to the two-dimensional image display element. In this case, however, the lens is symmetrical with respect to the light beam passing the center of the two-dimensional image display element, whereas the two-dimensional image display element itself has a higher contrast in only one direction, so that the light beam irradiated to the two-dimensional image display element is required to be at an angle.
A display device such as LCD or the like is ordinarily used as the two-dimensional image display element, and unlike the case when using a CRT, it is difficult to correct distortion of the projection lens because an LCD is driven by using a matrix of electrodes. That is, in the case of the CRT, the distortion of the projection can be relatively easily corrected by using a raster-shaped correction function such as a pin-cushion distortion correction or the like. On the other hand, in the case of the display device for performing dot matrix display such as an LCD, such a raster distortion correction is not ordinarily performed.
From the above situation, it is preferable to reduce the distortion aberration of the projection lens as much as possible. However, this is an obstacle to a wide-angle design of a projection lens and to achieving a long back focus.
That is, it is known that if telecentricity is provided to a projection lens while the wide-angle design and the long back focus are secured, this tends to increase the overall length of the lens and increase the lens diameter.
Further, since the back focus is insufficient and the incidence angle and emission angle of the light beam on the axis are acute in a wide-angle photographic lens for a single-lens reflex camera and a projection lens for a CRT-based projection television, there is no telecentricity and the light output is reduced.
Further, in the case of the projection display device using the construction of bending the optical path of the projection lens, it is known that if a so-called overall extension system to achieve the focus position by adjusting the relative distance between the projection lens and the screen is used as a focus adjusting system, the image center on the screen is deviated and, thus, is not suitable. Accordingly, in the projection display device, it is necessary to use a suitable focus adjusting method other than the overall extension system.